试用定语从句完成下列句子

I always don't know why he made such a mistake.
He is the man which/that I met at the exhibition.（关系代词which,that作宾语,也可以省略）
Is this the student to whom you lent the dictionary to?或Is this the student that/whom you lent the dictionary to?
He lives in the room whose windows face north.或He lives in the room of which the windows face north.
This is the place where/in which he was born.